Hayden Panettiere Found Dead by a Friend, 911 Call of a Female in 'Cardiac Arrest'

Hayden Panettiere was found dead by a friend inside a South Carolina home ... and Emergency Medical Service workers responded to a call of a person in "cardiac arrest" and an "unresponsive female," TMZ has learned.

Law enforcement sources tell us ... Hayden's friend called 911 at 1:51 pm Sunday to report that the actress was unresponsive and in cardiac arrest inside the residence in Greenville, SC. It's unclear if Hayden's on-again, off-again boyfriend Brian Hickerson placed the call.

We're told Greenville police officers and EMS workers quickly responded and paramedics attempted to resuscitate Hayden using "advanced cardiac life support." Our sources say this treatment builds on CPR by adding special medicines, breathing tubes, and heart rhythm tracking.

We're told paramedics also did chest compressions on Hayden, but nothing could bring her back to life and she was pronounced dead at 2:32 p.m.

Police say there were no initial signs of foul play and the circumstances surrounding Hayden's death did not appear to be suspicious. The Greenville County coroner will perform an autopsy Monday to determine a cause and manner of death.

Hayden began her career as a child star in TV and film, and was best known for her roles in the hit series "Heroes" and the musical drama "Nashville." She also landed major roles in other movies such as "Remember the Titans," "Raising Helen," "Ice Princess" and "Scream 4."

Hayden was 36.